The present invention relates to a current detection circuit for detecting the current flowing in a switching device, such as a MIS FET (metal insulator semiconductor field effect transistor), and a switching power supply using the same.
In recent years, switching power supplies have been used in various electronic apparatuses because of their high efficiency and high power conversion characteristics. In particular, in the case that quick transient response characteristics are requested, the current mode control system, which is not affected by the resonance frequency of an inductor and an output capacitor constituting such a switching power supply, is used. The current mode control system detects the current flowing in the inductor and controls the current to carry out output control, and requires a current detection circuit. In the case that a current detection device, such as a resistor, is used in a current detection circuit, a conduction loss occurs and its efficiency is reduced. Hence, a current detection circuit for detecting the current flowing in a switching device, such as a MOS FET (metal-oxide semiconductor field effect transistor), has been proposed as described in International Publication No. WO00/079682, for example.
FIG. 11 is a circuit configuration diagram showing a current detection circuit described as a conventional current detection circuit in International Publication No. WO00/079682. In FIG. 11, an output transistor 101 is formed of an N-channel MOS FET. The drain of this output transistor 101 is connected to a DC power supply 110, and the source thereof is connected to one terminal of a load 103. The other terminal of the load 103 is grounded. An auxiliary transistor 102 is an N-channel MOS FET, the drain thereof is connected to the DC power supply 110, and the source thereof is connected to the source of a compensation transistor 105. The ratio I101/I102 of the source current I101 of the output transistor 101 to the source current I102 of the auxiliary transistor 102 is designed to have a substantially constant value (hereafter, n=I101/I102) in the case that the potentials of the drain, gate and source of one of the two switching devices are made equal to those of the other, respectively. This can be realized by making the gate length of the output transistor 101 equal to that of the auxiliary transistor 102 and by setting the ratio of the gate width of the output transistor 101 to that of the auxiliary transistor 102 at n:1 in the case that the current detection circuit is produced so as to be built in a monolithic integrated circuit, for example. The compensation transistor 105 is a P-channel MOS FET, and the drain thereof is connected to one terminal of a current detection resistor 106. The other terminal of the current detection resistor 106 is grounded. A differential amplifier 104 detects the potential difference between the connection point P of the output transistor 101 and the load 103 and the connection point Q of the auxiliary transistor 102 and the compensation transistor 105. The differential amplifier 104 amplifies the detected potential difference and outputs the amplified voltage to the gate of the compensation transistor 105. A drive circuit 111 outputs a common drive signal to the gates of the output transistor 101 and the auxiliary transistor 102.
The operation of the conventional current detection circuit shown in FIG. 11 will be described below.
First, when the potential at the connection point P with respect to the potential at the connection point Q increases in the positive direction by virtue of the differential amplifier 104 and the compensation transistor 105, the potential at the connection point Q rises. Conversely, when the potential at the connection point P increases in the negative direction, the potential at the connection point Q lowers. Hence, the source potential (the potential at the connection point P) of the output transistor 101 becomes substantially equal to the source potential (the potential at the connection point Q) of the auxiliary transistor 102. Furthermore, the drain potential and the gate potential of the output transistor 101 are equal to those of the auxiliary transistor 102, respectively, as clearly understood from the circuit configuration. Hence, the potentials of the drain, gate and source of the output transistor 101 are equal to those of the auxiliary transistor 102, respectively. The ratio I101/I102 of the source current I101 of the output transistor 101 and the source current I102 of the auxiliary transistor 102 is thus maintained at the constant value n. In other words, the source current I102 of the auxiliary transistor 102 is I102=I101/n, a detection voltage Vs proportionate to the source current I101 of the output transistor 101 is generated across the current detection resistor 106. When it is assumed that the resistance value of the current detection resistor 106 is Rs, the detection voltage Vs is Vs=Rs·I101/n.
In the case that the conventional current detection circuit configured as described above is applied to the detection of the current flowing in a switching device in a switching power supply, an inductor and a rectifier circuit are connected to a load. In the case of a step-down converter wherein a synchronous rectifier circuit is used in a rectifier circuit, the current of the switching device is passed in the reverse direction in some occasions so that power is regenerated from the output to the input in order to promptly suppress overshoots occurred in the output, for example. However, the conventional current detection circuit configured as described above can detect only the current flowing from the output transistor to the load. In other words, the conventional current detection circuit configured as described above cannot detect the reverse current flowing in the switching device of the switching power supply.